For many years, researchers have been investigating methods for stabilizing 1-ascorbic acid (vitamin C), due to its beneficial properties. Indeed, 1-ascorbic acid has many known biological functions, such as the stimulation of collagen synthesis, the strengthening of skin tissue against external attack (UV radiation, pollution), depigmentation, activity against free radicals and the compensation for vitamin E deficiency. Some of these beneficial properties have been reported by England and Seifter in the article "The biochemical functions of ascorbic acid" (Ann. Rev. Nutri. (1986) 6:365-406).
However, due to its alpha-keto lactone structure, ascorbic acid is very sensitive to the influence of environmental parameters such as light, oxygen, and water. An unavoidable degradation of ascorbic acid in solution occurs over time due to its pH and the presence of trace metals.
This problem has been addressed in a variety of ways in the art. In order to reduce or delay the degradation of ascorbic acid in solution, U.S. Pat. No. 5,140,043 recommends stabilization by introducing ascorbic acid into aqueous-alcoholic solutions, formed of at least 80% water and having a pH below 3.5. These solutions are not usable in the cosmetic and/or pharmaceutical field because of a combination of pH, drying of the skin by the alcohol, and a lack of aesthetic "feel." Indeed, repeated application of these solutions may disrupt the equilibrium of the skin and may in particular irritate, or even burn, the skin.
Others have tried different ways to produce a stable ascorbic acid solution. B. R. Hajratwala, in "Stability of ascorbic acid", published in the Revue Sciences Pharmaceutiques on Mar. 15, 1905, discloses that ascorbic acid may be stabilized as an acidic aqueous solution by adding an oxyethylenated sorbitan ester surface-active agent. In particular, Hajratwala states that at pH3.4 and 25.degree. C., the addition of this agent reduced the rate of oxidation, and thus the rate of degradation, of ascorbic acid in solution. Hajratwala also discloses the use of a chelating agent such as ethylenediaminetetraacetic acid (EDTA) and packaging under nitrogen, in the absence of light, in order to enhance the stability of the aqueous ascorbic acid solution. However, such an acidic aqueous solution, applied to the skin, has the same drawbacks as those described above for acidic aqueous-alcoholic solutions. Furthermore, the stabilization obtained is still insufficient. Other ways of stabilizing ascorbic acid have been proposed, in particular by a coating technique (FR-A- 1,600,826) or by granulation of ascorbic acid (JP-A-53-127,819) for the agri-foods industry.
These techniques, while providing some stability, are, on the one hand, expensive and may, on the other hand, damage the ascorbic acid, for example during heating. Generally, these techniques lead to compositions of poor cosmetic acceptability, as in the case of granules. Consequently, none of the previous proposals have made it possible to overcome the technical problem associated with the instability of ascorbic acid in solution, in a form which is suitable for the cosmetic and/or dermatological fields and at a cost which is compatible with industrial requirements.
Accordingly, an object of the invention is to provide a method and formulation containing ascorbic acid that has improved stability.
Another object of the invention is to provide a method and formulation containing ascorbic acid that has proper aesthetic qualities for use in a dermatological or cosmetic product.
A further object of the invention is to provide a method and formulation containing ascorbic acid that is relatively inexpensive while providing the desire stability and aesthetic properties.
These and other objects and features of the invention will be apparent from the detailed description and the claims.